1. Field of the Invention
The present invention relates generally to a phase locked loop (PLL) circuit, and more particularly to a PLL circuit which is suitable for use in reproduction of a signal recorded on a disc-shaped recording medium.
2. Description of the Related Art
In techniques for reading data on a disc as a recording medium while rotating the disc, preformat data called "VFO" (Variable Frequency Oscillator) or "VFO-SYNC" (Variable Frequency Oscillator-Synchronization) has been conventionally used as a sequential repetitive data pattern for ensuring the reproduction of data recorded on the disc even if the rotation of the disc varies. The VFO is recorded on the disc together with main information data in a predetermined format. During a reading operation, in turn, a PLL provided in a reading system generates a clock signal to lock the PLL to the VFO data pattern, the clock signal being used for reproducing the information data.
With the foregoing configuration, since variations in the rotation of the disc causes simultaneous variations in a data waveform of the VFO data, the PLL is locked (or followed) to such a read data waveform susceptible to variations to thereby generate a suitable clock for the reproduction of similarly variable information data.
At present, a high density recording disc called "Digital Video Disc" (hereinafter called the "DVD") has been brought into a practical use. There are a variety of recording forms and reading forms for the DVD, one of which is a type called "DVD-RAM" (DVD-Random Access Memory).
An attempt has been made to apply the aforementioned information reproducing techniques using the VFO also to such DVD-RAM. However, the DVD-RAM employs a format which has a quite limited VFO field on the disc, in other words, uses a very short VFO data pattern, because this is redundant data, to increasing an area for recording essential information data so that high density recording is achieved. A PLL provided in a reading system, which is for reproducing a clock, must lock to the VFO data within the VFO field in a read signal. If the lock of the PLL to the VFO data cannot be carried out within the VFO field, incomplete reproduction of the information data will result. Thus, if the VFO data pattern is too short, it is more difficult to lock the PLL to the VFO data pattern, thereby causing possible deterioration in information reproducing performance.